Download Waves R` Us

Waves R‘ Us
What are Waves?
•  Waves: moving disturbances that transmit energy
without the physical transport of material
- waves in a pool or waves in a wheat field or waves
of people in a football field.
•  Waves have wavelength and wave speed
More on Waves?
•  Waves DOT NOT transport material in order to
transport energy
- waves in a pool just make the water move up and
down
-  this effect is best seen if you watch a fly resting
on the water or if you throw wood chips on the
water
Example: Water wave
• Water just moves
up and down
• Wave travels and
can transmit energy
(tsunami)
waves
•  all these waves travel through “something”
–  water waves: ocean surface
–  sound waves: air
–  slinky waves: slinky
–  string waves: string
•  this is called the “medium”
•  In pre-modern physics: no medium: no wave
Properties of waves
Frequency: Number of wave crests that pass a
given point per second
Period: Time between passage of successive
crests
Relationship:
Period = 1 / Frequency
Properties of waves
Wavelength: Distance between successive crests
Velocity: Speed at which crests move
- water and sound waves have different speeds
- electromagnetic waves travel in vacuum with
the speed of light (light is an electromagnetic wave)
Relationship: Velocity = Wavelength / Period
or
Velocity = Wavelength * Frequency
Visible Electromagnetic waves - Light
•  White light is a mixture of colors (previous slide)
•  Colors (seven total) are ranges of frequencies in the
electromagnetic spectrum
•  It is the human eye and brain that makes us perceive
‘colors’
•  White color splits into primary colors when it passes
through e.g. a prism or any other transparent material
(plastic, water etc)
- Rainbow is an example of white light splitting
Do all Waves need a Medium?
•  Many kinds of waves they need a material, matter, to propagate
- Water waves, sound waves, and so on, travel in a medium (water,
air respectively)
•  Electromagnetic waves need no medium. They travel in vacuum.
- Initially thought to travel through Ether, some sort of perfect
medium, but measurements exclude its existence
•  Created by accelerating charged particles (oscillating currents) and
other atomic processes
•  Electromagnetic (EM) waves:
•  Oscillating electric and magnetic fields normal to each other
•  Changing electric field creates magnetic field, and vice versa
What is the wave speed (c) of electromagnetic
waves?
c = 300,000 Km (186,411 miles) per second
This speed is very large, but still finite; it can
take light millions or even billions of years to
traverse astronomical distances
The Electromagnetic Spectrum
The Electromagnetic Spectrum
So sequence is: increasing frequency/energy, decreasing wavelength
Radio
Microwaves Infrared Visible UV X-rays Gamma -rays
Let there be light
•  “red” is agreed upon name for light with
wavelength at left end
•  “violet” is agreed name for light at right end
Red ‘feels’ warmer and Blue/Purple colder
In reality Red carries less energy than Blue !!!
(Watch this)
• No limit on wavelengths; different ranges have different
names
• Note opacity of atmosphere, it absorbs most radiation
with just a few exceptions like some radio waves and
visible light.
- That is why astronomy detectors in other ranges
need to be outside the atmosphere, in orbit.
-  Examples are Microwave, X-ray and Gamma-ray
detectors
Examples
Example: Radio bursts from the Sun, the Earth, and even from
Jupiter's ionosphere. The far right of this graph shows radio bursts
from the Sun caused by electrons that have been ejected into space
during solar flares moving at 20% of the speed of light.
Infrared
In both pictures the
left part is taken in the
Visual and the right in
the Infrared part of
the spectrum
The Sun is the dominant source of visible light that our
eyes perceive. Here you see a spectacular view of Sun’s
corona, during a total Solar eclipse, as it is shaped by its
strong magnetic fields
Visible
Ultraviolet (UV) light has shorter wavelengths than visible light.
Although UV waves are invisible to the human eye, some insects, such as
bumblebees, can see them. This is similar to how a dog can hear the
sound of a whistle just outside the hearing range of humans.
The Sun is a source of the full spectrum of ultraviolet radiation
UV
X-rays have much higher energy and much shorter wavelengths than
ultraviolet light, and scientists usually refer to x-rays in terms of their
energy rather than their wavelength.
X-rays have very small wavelengths, between 0.03 and 3 nanometers, so
small that some x-rays are no bigger than a single atom of many
elements
Can you see the filling of the tooth in the picture in the right?
X-rays
Gamma rays have the smallest wavelengths and the most energy of any
wave in the electromagnetic spectrum. They are produced by the
hottest and most energetic objects in the universe.
On Earth, gamma waves are generated by nuclear explosions, lightning,
and the less dramatic activity of radioactive decay.
Gamma-rays
IT IS ALL ABOUT LIGHT - OPTICS
Albert Einstein
I spent my life on the question “What is the photon”. Every
rascal thinks he knows but he deceives himself
The Particle/Wave nature of Light
Light-waves come in lumps! Photons
•  “photons” -- quanta (pieces) of light energy,
pieces of electromagnetic packets
•  very, very small lumps
Light/photons move on a straight line
•  So as light travel away from a source (lamp)
it can be though of moving on a straight line
(light rays)
•  Light waves across the electromagnetic spectrum
behave in similar ways.
•  When a light wave encounters an object, they are
either
• transmitted, (like through a piece of glass)
• reflected, (like in a mirror)
• absorbed,
• refracted, (from a prism)
• diffracted, or
• scattered
depending on the composition of the object and the
wavelength of the light.
•  Let’s examine some of these properties
Wave Properties- Absorption
•  Waves can be absorbed by a surface thus depositing its
energy
•  In this infrared image roads and dark surfaces are
‘hotter’ since asphalt etc absorb the energy of light and
heat up
Wave Properties- Scatter
•  Scattering occurs when light bounces off an object in a variety of
directions. The amount of scattering that takes place depends on the
wavelength of the light and the size and structure of the object
•  In the picture light scattering is used to detect the amounts and
distribution of volcanic ash
Reflection of Light
•  If light encounters a flat/polished surface part of
it changes direction, i.e. gets reflected
•  Example: light reflection from a mirror
•  RULE: Angle of incidence is equal to angle of reflection
–  Light follows the shortest distance path
θ1
θ2
θ1=θ2
Wave Properties- Reflection
In this image laser light was reflected on Moon’s surface
Different heights are shown as different colors